A variety of different protector circuits have been utilized to limit damage to data transmission lines, particularly telephone lines, due to lightning strikes and other voltage surges on the lines. The primary surge protector circuits and for data transmission lines customarily have utilized one or more "breakdown" or "crowbar" devices that are normally non-conductive but that are driven conductive by a voltage surge exceeding a predetermined breakdown voltage. Some of these devices are bidirectional, as is the case with carbon blocks, gas tubes, and solid-state triacs. Other, such as thyristors, may be unidirectional. The breakdown voltage is frequently about 250 volts; other breakdown levels may be selected.
Known primary surge protector circuits usually allow the voltage on a transmission line conductor to rise very rapidly when a lightning strike or other surge occurs on the line, until the breakdown voltage of the gas tube, triac, or other crowbar device goes conductive, and the impedance from the data conductor to ground or to some other reference potential reduces very rapidly. Indeed, the impedance to ground is usually reduced to near zero in a matter of nanoseconds, with the result that the voltage on the conductor is reduced to near zero potential with extreme rapidity.
The extremely rapid reduction in the impedance of the path from the data transmission conductor to ground through the gas tube or other breakdown device is undesirable because it does not provide for the dissipation of an appreciable amount of energy from the voltage surge during the time that the voltage on the struck conductor goes from the breakdown voltage of the crowbar device to ground potential. As a consequence, and because this energy must go somewhere, the primary protector circuit may not be as effective as desired in protecting equipment connected to the line at a location closely adjacent to the protector.
Voltage surges on data transmission lines that are well below the breakdown voltage of a primary protector circuit may nevertheless cause substantial damage, particularly in those instances when the surge entails a substantial energy content. This is also true of some voltage surges that would be of sufficient amplitude to drive the primary protector circuit to conduction but that are of extremely short duration. As a consequence, a primary protection circuit is frequently paired with a secondary protector circuit that employs one or more normally non-conductive threshold device that become conductive when stressed above a given threshold voltage that is still well below the breakdown voltage for the primary protector. Typically, for a two hundred fifty volt primary breakdown voltage the threshold voltages in the secondary protector may be in a range of about ten to seventy volts, always in series with a resistor or capacitor.
Some rather effective secondary protectors have been constructed as R/C filters; the filter resistance is in series in the data conductor, the capacitance in a shunt circuit. But these secondary protectors present appreciable problems in meeting safety standards, due to the presence of the resistance in the data signal conductors, and hence are unsatisfactory in some applications. The problem can be alleviated by use of positive thermal coefficient resistors, but this expedient is rather costly.
A common technique has been to provide a composite surge protection apparatus, incorporating primary and secondary protection circuits in one package. This is convenient with respect to space requirements and cost, but is not particularly desirable from the standpoint of effective overall protection.